Method for the determination of dissolved carbon in water

ABSTRACT

Apparatus and methods for the measurement of total organic carbon and total inorganic and organic content of water are described. A novel combination of an acidification module, and inorganic carbon removal module based on a carbon dioxide selective gas permeable membrane, and oxidation reaction system which incorporates in-situ generation of oxidizing agents, coupled with catalyzed photo-oxidation of organic compounds to form carbon dioxide, and a high sensitivity, conductometric detector employing a carbon dioxide selective gas permeable membrane permits on-line measurement of the total organic carbon content of water streams.

This is a divisional application of Ser. No. 07/487,720, filed Mar. 2,1990, now U.S. Pat. No. 5,132,094.

FIELD OF THE INVENTION

The present invention relates to an improved method and apparatus forthe determination of the total concentration of organic and/or inorganiccarbon compounds in aqueous process streams and in bulk solutions.Particularly, the method of the present invention includes theacidification of an aqueous sample stream, the in-situ generation ofoxidizing agents--including oxygen, hydrogen peroxide andperoxydisulfate or persulfate ion--used in conjunction withsemiconductor-catalyzed photo-oxidation of organic compounds to formcarbon dioxide, and the sensitive and selective detection of carbondioxide utilizing a gas permeable membrane and conductometric detection.

BACKGROUND OF THE INVENTION

The measurement of the total organic carbon (TOC) concentration andtotal carbon concentration in water has become a standard method foraccessing the level of contamination of organic compounds in potablewaters, industrial process waters, and municipal and industrial wastewaters. In addition to widespread terrestrial applications, themeasurement of TOC is one of the primary means of determining the purityof potable and process waters for manned space based systems includingthe space shuttle, the proposed space station and for future mannedexplorations of the moon and other planets.

A variety of numerous prior art approaches for measuring the totalorganic carbon content of water have been proposed. For example, SeeU.S. Pat. No. 3,958,941 of Regan; U.S. Pat. No. 3,224,837 of Moyat; U.S.Pat. No. 4,293,522 of Winkler; U.S. Pat. No. 4,277,438 of Ejzak; U.S.Pat. Nos. 4,626,413 and 4,666,860 of Blades et. al.; and U.S. Pat. No.4,619,902 of Bernard.

Representative of the devices described in these references are themethods disclosed in U.S. Pat. No. 3,958,941 of Regan. In Regan anaqueous sample is introduced into a circulating water stream that flowsthrough a reaction chamber where the sample is mixed with air andexposed to ultraviolet (U.V.) radiation to promote the oxidation oforganic compounds to form carbon dioxide. The carbon dioxide formed inthe reaction chamber is then removed from solution by an air strippingsystem and introduced into a second chamber containing water that hasbeen purified to remove ionic compounds. The conductivity of the waterin the second chamber is measured, and any increase in conductivity isrelated to the concentration of carbon dioxide formed in the firstreactor. The conduction measurement can be used, therefore, to determinethe concentration of organic compounds in the original sample.

The Regan device is slow, cannot be used for the continuous monitoringof TOC concentration in aqueous streams, cannot be scaled down withoutincreasing interference from NO₂, CO₂ and H₂ S to unacceptable levels,and is generally unsatisfactory. In addition, Regan does not disclosethat an aqueous solution of acid must be added to the sample stream toreduce the pH to a value of less than about 4 to insure reasonableremoval rate of carbon dioxide using the air stripping system described.The oxidation method disclosed by Regan is unsatisfactory for themeasurement of refractory compounds, particularly urea. In Regan, anaqueous sample of 20 to 100 mL containing 0.5 mg/L organic carbon isrequired to generate sufficient carbon dioxide for accurate detection,thus limiting the utility of the device for the measurement of sub-partper million levels of TOC in smaller sample sizes. Finally, in practice,the Regan system requires frequent recalibration--typically once perday--due to variations in background conductivity. Also, theconcentration of total organic carbon in the calibration standard mustbe approximately equal to the concentration of organic carbon in thesample. Because of this, recalibration is required when analyzingaqueous samples containing higher or lower levels of organic carbon whencompared with the calibration standard.

An improved method and apparatus for the measurement of organic contentof aqueous samples is that disclosed in U.S. Pat. No. 4,277,438 ofEjzak. Ejzak describes a multistage reactor design which provides forthe addition of oxygen and a chemical oxidizing agent, preferably sodiumpersulfate, to the aqueous sample stream prior to oxidation of thestream using ultraviolet radiation in a series of reactors. Ejzak alsodescribes the use of an inorganic carbon stripping process--beforeoxidation of the organic carbon--that includes the addition ofphosphoric acid to the sample stream. After oxidation, the sample streamis passed into a gas-liquid separator where the added oxygen acts as acarrier gas to strip carbon dioxide and other gases from the aqueoussolution. In the preferred embodiment, the gas stream is then passedthrough an acid mist eliminator, a coalescer and salt collector, andthrough a particle filter prior to passage into an infrared (IR)detector for the measurement of the concentration of carbon dioxide inthe gas stream.

The methods and apparatus disclosed by Ejzak provide improvements overthe teachings of Regan, however, the Ejzak device requires extensivemanual operation and is also generally unsatisfactory. The Ejzak devicerequires three external chemical reagents; oxygen gas, aqueousphosphoric acid and an aqueous solution of sodium persulfate. Both thephosphoric acid and persulfate solutions must be prepared at frequentintervals by the operator due to the relatively high rate ofconsumption. The Ejzak device requires dilution of the sample if thesolution contains high concentrations of salts in order to insurecomplete oxidation of the sample and to eliminate fouling of theparticle filter located prior to the IR carbon dioxide detector. As withRegan, relatively large sample sizes are required--typically 20 mL ofsample for accurate measurement at 0.5 mg/L total organic carbon--andthe carbon dioxide formed in the oxidation chamber is removed using agravity dependent technique that cannot be easily used in space-basedoperations.

Another improved method and apparatus for the measurement of totalorganic carbon in water is disclosed in U.S. Pat. No. 4,293,522 ofWinkler. In Winkler, an oxidizing agent, molecular oxygen, is generatedin-situ by the electrolysis of water. Organic compounds are subsequentlyoxidized to form carbon dioxide by the combination of U.V. radiation andthe in-situ generated oxygen. The irradiation and electrolysis processesare both accomplished in a single oxidation chamber. Winkler does notteach that the aqueous sample stream be acidified to assist in theremoval of carbon dioxide from solution, and in fact teaches against theuse of acid. Therefore, this method and apparatus cannot be used for themeasurement of organic compounds in basic aqueous samples. The oxidationchamber of Winkler uses a solid electrolyte to separate the twoelectrodes employed for the electrolysis of water. The solid electrolytedescribed by Winkler is composed of an organic polymer which, underexposure to oxygen, ozone and U.V. radiation, will undergo oxidation toform carbon dioxide; therefore, resulting in unacceptable backgroundlevels of organic compounds in the sample stream, particularly at loworganic compound concentrations.

Winkler also describes a conductometric carbon dioxide detection systemwherein the sample stream exiting the oxidizing chamber is held in anequilibrating relationship to a stream of deionized water. The twoflowing streams are separated by a gas permeable membrane that allowsthe concentration of carbon dioxide to equilibrate between the streams.The concentration of the carbon dioxide is thereby determined bymeasuring the conductance of the deionized water stream. However, theuse of two flowing streams introduces operating parameters into thedetection process that require frequent calibration adjustments.

Another example of the prior art is that disclosed in U.S. Pat. No.4,619,902 of Bernard, which teaches the oxidation of organic compoundsto form carbon dioxide using persulfate oxidation at elevatedtemperatures--typically 20° to 100° C.--in the presence of a platinummetal catalyst. Bernard recognizes that the materials used in theconstruction of instrumentation for the determination of total organiccarbon in water can contribute organic compounds to the sample duringthe measurement process, and teaches that inert materials such as PTFEmust be used to reduce this background from the measurement. As with thepreviously mentioned disclosures, a gas stripping technique is employedto collect the formed carbon dioxide, and measurement is made using IRspectrometry. Bernard also recognizes that aqueous solutions of sodiumpersulfate are not stable due to auto-degradation of the reagent.

An improved system for the measurement of organic compounds in deionizedwater is disclosed in U.S. Pat. No. 4,626,413 of Blades and Godec. Theapparatus described by Blades and Godec is based on direct U.V.oxidation of organic compounds to form carbon dioxide which is measuredby using conductometric detection. In the apparatus described in Bladesand Godec, the oxidation of some organic compounds from such strongacids such as HCl, H₂ SO₄ and HNO₃ which interfere with theconductometric method. The Blade device is also limited to themeasurement of total organic compounds in deionized water and cannot beused for samples containing ionic compounds other than bicarbonate ion.

In U.S. Pat. No. 4,209,299 of Carlson, it is disclosed that theconcentration of volatile materials in a liquid can be quantitativelydetermined by transferring the desired material through a gas permeablemembrane into a liquid of known conductivity, such as deionized water.The Carlson device is demonstrated for the measurement of a number ofvolatile organic and inorganic compounds, but Carlson does not suggestthe combination of this process in conjunction with a carbon dioxideproducing reactor.

The use of aqueous solutions of persulfate salts for the oxidation oforganic compounds is widely known. Smit and Hoogland (16 ElectrochimaActa, 1-18 (1971)) demonstrate that persulfate ions and other oxidizingagents can be electrochemically generated.

In U.S. Pat. No. 4,504,373 of Mani et. al., a method for theelectrochemical generation of acid and base from aqueous salt solutionsis disclosed.

In electrochemical reactions in aqueous solutions, a common reductionproduct is hydrogen gas. Because of its flammability, the hydrogenpresents a potential hazard in devices using electrochemical techniques.The interaction of hydrogen gas in aqueous solutions and palladium metalis well known (e.g., F A. Lewis, "The Palladium Hydrogen System,"Academic Press, 1967, London, incorporated herein by this reference) andthe use of palladium offers a potential solution to the generation ofhydrogen in electrochemical reactions by selective removal and disposalof the hydrogen.

SUMMARY OF THE INVENTION

Recognizing the need for accurate on-line measurement of theconcentration of total organic and inorganic carbon compounds in aqueousstreams and the problems and limitations of existing methods andapparatus used for these measurements, the present invention provides anovel method and apparatus which eliminates or overcomes these problems.Specifically, the present invention incorporates four significantadvantages; 1) the carbon dioxide detector described herein utilizes aselective gas permeable membrane for the transport of carbon dioxideeither from the oxidized or unoxidized sample stream into a secondaqueous solution where the sensitive detection of carbon dioxide isaccomplished using conductometric measurement, thus eliminating the useof a gas stripping apparatus, 2) in-situ generation of oxidizing agentsincluding persulfate ions, hydrogen peroxide and molecular oxygen, thuseliminating the need for the introduction of gases and unstable chemicalreagents, 3) an in-line acidification module which permits accuratedetermination of the organic content of aqueous samples over a widesample pH range, and 4) the incorporation of an oxidation catalyst toinsure rapid and complete photo-oxidation of organic compounds.

In one embodiment of the present invention, an aqueous sample stream ispassed through a filter to remove any particulate matter, and passedinto a acidification module for the introduction of a suitableconcentration of acid to cause a reduction in the pH of the solution toa pH of less than 4. Inorganic carbon species--primarily carbonate andbicarbonate ions--are reacted with the acid to form carbon dioxide,while organic compounds remain unreacted.

The effluent of the acidification module is directed into an inorganiccarbon removal module comprised of a carbon dioxide selective gaspermeable membrane or a non-selective gas permeable membrane, whichseparates the acidified sample stream from a second aqueous stream inwhich the pH of the stream has been raised to a pH of greater than 10 byaddition of a suitable base. The carbon dioxide formed from the reactionof inorganic carbon species with the acid will selectively diffuseacross the gas permeable membrane into the basic aqueous stream wherethe carbon dioxide will be converted to ionic species (carbonate orbicarbonate) for subsequent disposal.

The acidic and basic streams used in the acidification module andinorganic carbon removal modules may be composed of aqueous solutions ofsuitable acids and bases or alternatively, an aqueous salt solution canbe passed through a system incorporating a bi-polar membrane (see U.S.Pat. No. 4,504,373, specifically incorporated herein by this reference)for the in-situ generation of an acidic stream, a basic stream, and adepleted salt stream.

The effluent of the inorganic carbon removal module is then directedinto a U.V. oxidation module which incorporates either direct U.V.oxidation using short wavelength U.V. radiation, semiconductor catalyzedU.V. oxidation using short wavelength U.V. radiation, or U.V. oxidationin the presence of oxygen and or other oxidizing agents such aspersulfate, which are generated in-situ by the electrolysis of water andother chemical reagents such as sodium sulfate. In the U.V. oxidationreactor, organic compounds are converted to carbon dioxide. A palladiumcathode system can be employed in the electrolysis apparatus for theremoval of any hydrogen generated during the electrolysis of water.

The carbon dioxide formed in the photoreactor is then sensitivelymeasured using a novel carbon dioxide sensor. The sensor is comprised ofa carbon dioxide selective gas permeable membrane which separates theacidified sample stream from a deionized water reservoir. The deionizedwater is continuously generated by means of a mixed bed ion exchangeresin. Alternatively, deionized water can be supplied from a sourceexternal to the apparatus described in the present invention.

In the basic measurement cycle, a fresh pulse of deionized water isintroduced into the deionized water side of the gas permeable membraneand a shut-off valve actuated to stop the flow of deionized water. Theeffluent of the photoreactor continuously flows on the opposite side ofthe membrane. The carbon dioxide formed in the photoreactor from theoxidation of organic compounds diffuses across the gas permeablemembrane until the concentration of carbon dioxide in the two aqueousstreams is substantially the same. As the carbon dioxide enters thedeionized water, the carbon dioxide will dissolve in the water and causean increase in the conductivity of the aqueous solution. Afterequilibrium has been established (typically about 5 min.), a fresh pulseof deionized water is introduced to sweep the equilibrated solution intoa conductivity cell in order to measure the increase in theconcentration of ionic species.

The increase in conductivity observed in the deionized water can bedirectly related to the concentration of carbon dioxide in the samplestream and hence the level of organic compounds originally present inthe sample stream.

In an alternate embodiment of the present invention, the apparatus canbe modified to permit measurement of the total carbon content of thesample and the total inorganic carbon content of the sample. In thisembodiment, the inorganic carbon removal module is replaced with twothree-way valves which permit the acidified sample stream to bypass thephoto-reactor. The concentration of total inorganic carbon in theaqueous sample is determined when the photoreactor is bypassed and theacidified sample stream proceeds directly into the sample side of thegas permeable membrane component of the carbon dioxide sensor. Asdescribed above, equilibration of the carbon dioxide present in thesample stream, due to the reaction of inorganic carbon species with asuitable acid, will cause an increase in the conductivity on thedeionized water side of the sensor and this increased conductivity canbe measured by using a conductivity cell and directly related to theconcentration of inorganic carbon species present in the aqueous sample.

After measurement of the total inorganic carbon concentration, the twothree-way valves are repositioned to allow the acidified aqueous streamto pass through the photoreactor for the oxidation of organic compoundsto form carbon dioxide. In this operational mode, the carbon dioxidesensor component will determine the total carbon content of the samplestream (total inorganic and organic carbon concentrations). The level oforganic compounds in the sample is then determined from the differencebetween the total carbon concentration and the previously measured totalinorganic carbon concentration.

In a third embodiment of the present invention, the use of the streamsplitter and three-way valve is replaced with a flow-through systemwhich incorporates an external switch for the electrical connections tothe U.V. photoreactor. Without a source of U.V. radiation, organiccompounds in the acidified sample stream will not be converted to carbondioxide, while inorganic carbon species will react with the added acidto form carbon dioxide, which is detected by the carbon dioxide sensor.After the measurement of total inorganic carbon, the electrical power tothe source of ultraviolet radiation is restored, resulting in thecoversion of organic compounds to carbon dioxide. After irradiation, thelevel of carbon dioxide in the sample stream, as measured by the carbondioxide sensor, will be proportional to the level of total carbonspecies (organic and inorganic) present in the sample. The level oftotal organic carbon in the sample stream is then computed from thedifference between the detector response with the U.V. lamp on (totalcarbon) and the lamp off (total inorganic carbon).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram depicting an embodiment of the presentinvention for the on-line measurement of total organic carbonconcentrations with removal of inorganic carbon compounds.

FIG. 2 is a block diagram depicting an embodiment of the presentinvention for the on-line measurement of total organic concentrationsemploying a bipolar membrane module for the generation of acid and basefrom an aqueous salt solution.

FIG. 3 is a block diagram depicting an embodiment of the presentinvention for the on-line measurement of both total organic andinorganic carbon concentrations, employing a stream splitting technique.

FIG. 4 is a block diagram depicting an embodiment of the presentinvention for the on-line measurement of both total organic andinorganic carbon concentrations without stream splitting.

FIG. 5 is a schematic representation of the semiconductor-packedphotoreactor module of the present invention.

FIG. 6 is a schematic representation of the semiconductor-packedphotoreactor module of the present invention incorporating sections ofpalladium tubing for hydrogen removal.

FIG. 7 is a schematic representation of an in-line electrolytic oxygengenerator and semiconductor-packed photoreactor module of the presentinvention.

FIG. 8 is a schematic representation of an in-line electrolysis modulefor the generation of persulfate, hydrogen peroxide and oxygen and foruse with semiconductor-packed photoreactor module of the presentinvention.

FIG. 9 is a schematic representation of an off-line electrolysis modulefor the generation of high concentrations of persulfate for use with thesemiconductor-packed photoreactor module of the present invention.

FIG. 10 is a schematic representation of the combined in-situelectrolysis module and photo-oxidation reactor of the presentinvention.

FIG. 11 is a schematic representation of the carbon dioxide sensorcomponent of the present invention.

FIG. 12 is a schematic representation of the carbon dioxide sensorcomponent of the present invention with an internal conductivity sensor.

FIG. 13 is a representation of the output from the conductivity sensorduring a measurement cycle.

FIG. 14 is a plot of the logarithm of the response of the conductivitysensor versus the logarithm of the concentration of carbon in theaqueous sample.

FIG. 15 is a schematic of the acidification module component of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The measurement of the total organic content of aqueous samples hasbecome a standard technique for determining the quality of potablewater, industrial process water and industrial and municipal wastewaters.

The determination of the organic content of water samples is mostcommonly achieved by oxidation of the organic constituents to carbondioxide using chemical oxidizing agents, U.V. radiation, or acombinations of these methods and subsequent detection of the carbondioxide using IR spectroscopy or by conductometric or potentiometrictechniques. The present invention is an improved process and apparatusfor determining concentration levels of total organic and inorganiccarbon compounds in aqueous samples.

A block diagram of one embodiment of the present invention is shown inFIG. 1. An aqueous sample inlet opening 10 is in communication with aparticle filter 12 for the removal of particulate matter that may besuspended in the aqueous sample stream. A filter outlet conduit 14 is influid communication with the inlet of an acidification module 16. Thewater sample inlet of the acidification module 16 is in communicationwith a hollow acid permeable membrane (not shown) and permits passage ofthe sample stream through the inside of the hollow membrane. An acidreservoir 18 and acid inlet conduit 20 is in communication with a secondinlet to the acidification module 16 which permits passage of the acidsolution only around the outside of the hollow membrane. The flow rateof the aqueous acid from the reservoir 18 is maintained at a flow ratesufficient to cause diffusion of acid across the hollow membrane, andcause a reduction in the pH of the aqueous sample stream to a pH of lessthan about 2. The outlet of the hollow membrane is in communication withthe aqueous sample outlet conduit 24 of the acidification module 16 anda second outlet conduit 26 is in communication with the area exterior tothe hollow membrane that permits passage of the depleted aqueous acidsolution to a tee 27 and, via conduit 28, to a suitable waste container.

The aqueous outlet conduit 24 from the acidification module is incommunication with the aqueous sample inlet of the inorganic carbonremoval module 32, which contains a gas permeable membrane 34 positionedsuch that the flowing aqueous sample stream passes on one side of thegas permeable membrane. An aqueous base reservoir 36 and base inletconduit 38 is in communication with a second inlet to the inorganiccarbon removal module 32, which is positioned such that the aqueous basestream passes on the opposite side of the gas permeable membrane fromthat of the aqueous sample. The flow of the aqueous base iscountercurrent to that of the aqueous sample. The aqueous samplesolution passes by means of the inorganic carbon removal module outletconduit 42 to the aqueous sample inlet of the U.V. oxidation reactor 46.A second inorganic carbon removal module outlet 48 permits passage ofthe depleted aqueous base solution to a tee 27 via conduit 28 to asuitable waste container 29. The mixing of the deleted acid and basesolutions in tee 27 minimizes any potential problems in the disposal ofthe waste streams.

A detailed description of the components of the U.V. oxidation reactor46 are given below and shown in FIGS. 5-10. The U.V. oxidation moduleoutlet conduit 52 is in communication with the aqueous sample inlet ofthe carbon dioxide sensor 56, which contains a gas permeable membrane 58positioned such that the flowing aqueous sample stream passes on oneside of the gas permeable membrane. A deionized water module 60 is incommunication via the deionized water module outlet conduit 62 with thedeionized water inlet of the carbon dioxide sensor 56 and the inlet ispositioned to permit passage of deionized water on the opposite side ofthe gas permeable membrane from that of the aqueous sample stream. Arelatively thin layer of deionized water (approximately 0.005") ismaintained on the deionized water side of the gas permeable membrane tofacilitate rapid analysis times.

The deionized water module consists of a mixed bed of anion and cationion exchange resins 66 in communication via a conduit 68 with acirculating pump 70 which is in communication via a conduit 72 to a tee74. One outlet of the tee 74 is in communication via conduit 76 with asolenoid shut-off valve 78, and the other outlet of the tee is incommunication via a conduit 80 to a flow restrictor 82. The outlet ofthe solenoid shut-off valve is in communication via the deionized wateroutlet conduit 62 with the deionized water inlet of the carbon dioxidesensor 56. The outlet of the flow restrictor 82 is in communication viaa conduit 84 to one inlet of a second tee 86 and the outlet of the teeis in communication via a conduit 88 to the inlet of the ion exchangeresin bed 66.

The deionized water outlet of the carbon dioxide sensor 56 is incommunication via a conduit 92 to the inlet of a micro-conductivitysensor 94. The outlet of the micro-conductivity sensor 94 is incommunication via a conduit 96 to the other inlet of the second tee 86.The aqueous sample outlet of the carbon dioxide sensor 98 is incommunication with the inlet of a peristaltic sampling pump 100, and theoutlet of the sampling pump is connected via a conduit 102 to a suitablewaste container 104. The micro-conductivity sensor 94 is connected to asuitable power supply (not shown) and the electrical output from themicro-conductivity sensor is connected to the control and signalelectronics module 106.

The control and electronic module 106 is comprised of a computer orcomparable electronic device which is capable of controlling thevoltages and currents to all of the electrical components of the presentinvention, actuation of valves and switches in a pre-determined timedsequence, processing of the electrical signal from themicro-conductivity sensor and the calculation of total organic carbonconcentration, total carbon concentration and total inorganic carbonconcentration from output of the conductivity sensor.

In an alternate embodiment of the present invention, shown in FIG. 2,the acid reservoir 18 and the aqueous base reservoir 36 are replacedwith an acid/base generation module 108 which consists of an aqueoussalt reservoir 110 which is in communication via conduit 112 with anelectrodialysis system (not shown) which incorporates bipolar membranes,anion and cation ion exchange membranes and an electrical power supplyfor the production of separate streams of an aqueous acid, and aqueousbase, and a depleted salt solution. The in-situ generated aqueous acidstream is in communication with the acid inlet of the acidificationmodule via an acid inlet conduit 114. The in-situ generated aqueous basestream is in communication with the base inlet 40 of the inorganiccarbon removal module via a base inlet conduit 116. The depleted saltsolution is connected via a conduit 118 to a suitable waste container29.

In operation of the present invention as described in FIGS. 1 and 2, theperistaltic sampling pump 100 withdraws an aqueous sample via the sampleinlet opening 10, at a desired flow rate of approximately 50 to 100microliters per minute into the acidification module 16. Aqueous acid,for example 3M phosphoric acid or 3M sulfuric acid, from the acidreservoir 18 or from the acid/base generation module 108 is passedthrough the acidification chamber, exterior to the hollow membrane, at aflow rate of approximately 5 uL/min. As the aqueous sample flows throughthe hollow membrane, some acid from the exterior of the membrane willdiffuse into the aqueous sample and result in a decrease in the pH ofthe aqueous sample stream. The desired pH of the aqueous sample streameffluent of the acidification module is a pH of less than about 4.

After acidification, the aqueous sample stream enters the inorganiccarbon removal module 32 via the aqueous sample inlet 30. Aqueous base,for example 3M sodium hydroxide, from the aqueous base reservoir 36 orfrom the acid/base generation module 108 is passed on one side of a gaspermeable membrane 34, while the aqueous sample is passed on theopposite side. Carbon dioxide produced from the reaction of inorganiccarbon species with the acid added to the aqueous sample stream in theacidification module 16 rapidly diffuses across the gas permeablemembrane and into the aqueous base stream where it is converted intoionic species. The gas permeable membrane 34 is constructed of amaterial that will permit diffusion of carbon dioxide and otherinorganic gases, but will not permit diffusion of organic acids andother volatile organic compounds.

The aqueous sample stream, after removal of the inorganic carboncompounds, enters the U.V. oxidation reactor 46, in which, using themethods and apparatus described below, organic compounds are convertedto carbon dioxide and other products.

The aqueous sample stream effluent of the U.V. oxidation reactor 46 isdirected via conduit 52 into the aqueous sample inlet of the carbondioxide sensor 56, out through the aqueous sample outlet of the carbondioxide sensor 98 through the peristaltic sample pump 100 to a suitablewaste container 104.

A continuous supply of deionized water is produced in the deionizedwater module 60 by passing an aqueous stream of water through the mixedbed ion exchange resins 66 by means of the circulating pump 70 with thesolenoid valve 78 in the closed position.

In the measurement cycle of the carbon dioxide sensor 56, the solenoidvalve 78 is switched to the ON position to introduce a sample ofdeionized water via conduit 62 into the deionized water inlet of thecarbon dioxide sensor 56. After a period of time (generally about 40 to100 seconds), the solenoid valve 78 is returned to the OFF position. Asthe sample stream passes on one side of the gas permeable membrane 58 ofthe carbon dioxide sensor 56, the carbon dioxide formed in the U.V.oxidation module 46 will diffuse across the gas permeable membrane intothe deionized water sample on the opposite side of the membrane, wherethe carbon dioxide will be converted into ionic species. After a shortperiod of time (generally about 5 min.) an equilibrium will beestablished between the concentration of carbon dioxide in the flowingaqueous sample stream and the deionized water sample across the gaspermeable membrane.

After this equilibration period, the solenoid valve 78 is switched tothe ON position and the deionized water sample is passed into themicro-conductivity cell 94 by means of the circulating pump 70. Theincrease in conductivity caused by the presence of ionic species formedfrom carbon dioxide is measured by the micro-conductivity cell 94 andassociated control and signal module 106. The observed increase in theconductivity of the deionized water sample can be directly related tothe concentration of carbon dioxide present in the aqueous samplestream, and hence, the level of organic compounds present in the aqueoussample stream by known means.

As the conductivity of the deionized water sample is being determined,the equilibration period for the next measurement cycle is underway.Thus, in the present invention, the measurement of the organiccomposition of an aqueous sample stream can be determined about everyfive minutes, or at any longer desired measurement interval.

An additional embodiment of the present invention is illustrated in FIG.3 for the determination of both total organic and total inorganic carbonconcentration in an aqueous sample stream. In this embodiment of theinvention, the inorganic carbon removal module 32 is replaced with asample stream splitter 122. The aqueous sample effluent of theacidification module 16 is in communication with sample stream splitter122 via conduit 124.

Approximately one-half of the aqueous sample stream is passed from theoutlet of the sample stream splitter 122 to the aqueous sample inlet ofthe U.V. oxidation reactor 46 via conduit 126. The remainder of theaqueous sample stream from the outlet of the sample stream splitter 122is in communication via conduit 128 with the inlet of the delay tubingcoil 130. The outlet of the U.V. oxidation reactor 46 is incommunication with the total carbon three-way valve 132 via conduit 134and the outlet of the delay tubing coil 130 is in communication with asecond, total inorganic carbon three-way valve 136 via conduit 138.

One outlet of the total carbon three-way valve 132 is in communicationwith one inlet of the carbon dioxide sensor inlet tee 140 via conduit142, and the second outlet of the three-way valve is in communicationvia conduit 144 to one inlet of the pump inlet tee 146. Similarly, oneoutlet of the total inorganic carbon three-way valve 136 is incommunication with the carbon dioxide inlet sensor tee 140 via conduit148, and the second outlet of the three-way valve is in communicationwith the pump inlet tee 146 via conduit 150.

The outlet from the carbon dioxide inlet tee 140 is in communicationwith the aqueous sample stream inlet of the carbon dioxide sensor 56 viaconduit 152 and the aqueous sample outlet of the carbon dioxide sensor56 is in communication via conduit 154 with the inlet of the peristalticsampling pump 156.

The outlet of the pump inlet tee is also in communication via conduit158 with the inlet of the peristaltic pump. In contrast with theapparatus described in FIGS. 1 and 2, the peristaltic sampling pump 156employed in this embodiment of the invention is used to sample twoseparate aqueous streams simultaneously, the aqueous outlet of thecarbon dioxide sensor 56 via conduit 154 and the outlet of the pumpinlet tee 146 via conduit 158. The pump outlet from both aqueous samplestreams is passed through conduits 160 and 162 to a suitable wastecontainer 164.

In operation, this embodiment of the invention employs to separatemeasurement cycles; the measurement of the total inorganic carbonconcentration of the aqueous sample and the measurement of the totalcarbon concentration (total organic and total inorganic carbon) of theaqueous sample. The total organic carbon concentration of the sample isthen computed from the difference between these two measurements. Asdescribed above, the peristaltic sampling pump 156 is used to draw theaqueous sample from the sample inlet 10, through particle filter 12 andthrough the acidification module 16. The aqueous sample then enters thesample stream splitter 122 which diverts approximately equal liquidflows through conduits 126 and 128.

In the total inorganic carbon measurement cycle, the total inorganiccarbon three-way valve 136 is positioned such that the aqueous samplestream flows through conduit 148 to the carbon dioxide sensor inlet tee140, with no liquid flow passing through conduit 150. The total carbonthree-way valve 132 is positioned such that the aqueous sample streamflows through conduit 144 to the peristaltic pump inlet tee 146, thusbypassing the carbon dioxide sensor 56.

As described above, a flow of deionized water from the deionized watermodule 60 is introduced into the deionized water inlet of the carbondioxide sensor 56 by positioning the solenoid valve 78 in the ONposition and the flow of deionized water terminated by positioning thesolenoid valve 78 in the OFF position. Carbon dioxide formed from thereaction of inorganic carbon species with the acid from theacidification module 16 will rapidly diffuse across the gas permeablemembrane 58 of the carbon dioxide sensor 56 resulting in an increase inthe conductivity of the deionized water, which is subsequently measuredby the micro-conductivity sensor 94. This increase in conductivity canbe directly related to the concentration of inorganic carbon species inthe aqueous sample by known means.

After the measurement of total inorganic carbon is complete (generallyabout 5 min.), the apparatus is reconfigured for the measurement of theconcentration of total carbon compounds in the aqueous sample. The totalcarbon three-way valve 132 is positioned to permit flow of the aqueoussample via conduit 142 go to the inlet tee of the carbon dioxide sensor140, with no liquid flow through conduit 144. The total inorganic carbonthree-way valve 136 is positioned to permit flow of the aqueous samplevia conduit 150 to the peristaltic pump inlet tee 146.

As the aqueous sample stream passes through the acidification module 16,inorganic carbon species will be converted to carbon dioxide, whileorganic compounds remain unreacted. In the U.V. oxidation module 46, theorganic compounds will be converted to carbon dioxide, thus the level ofcarbon dioxide in the aqueous sample stream passing through the carbondioxide sensor 56 will be directly proportional to the concentration ofboth organic carbon compounds and inorganic compounds in the originalsample.

The measurement of the total carbon content of the aqueous sample streamentering the carbon dioxide sensor 56 is conducted in the same manner asdescribed above. After measurement of the total carbon concentration andtotal inorganic carbon concentration, the total organic carbonconcentration is computed as the difference between these two values.

In this embodiment of the invention, the concentration of total carbon,total inorganic carbon, and total organic carbon can generally bedetermined approximately every ten minutes or at longer intervals ifdesired.

In another embodiment of the invention, the concentration of totalcarbon compounds, total organic carbons and total inorganic carboncompounds is determined using the apparatus shown in FIG. 4. In thisembodiment, the sample stream splitter 122 is replaced with a conduit166 which permits passage of the aqueous sample from the outlet of theacidification module 16 to the inlet of the U.V. oxidation module 46.The U.V. oxidation module 46 is equipped with an electrical power switch168. The aqueous sample stream outlet of the U.V. reactor module 46 isin communication with the aqueous sample inlet of the carbon dioxidesensor 56 via conduit 170.

In operation, the peristaltic sampling pump 100 is used to withdraw theaqueous sample via the sample inlet 10, through the particle filter 12and acidification module 16 as described above. For the measurement oftotal inorganic carbon compounds in the sample, the electrical power tothe U.V. oxidation reactor 46 is discontinued by positioning theelectrical power switch 168 in the OFF or OPEN position. Under theseconditions, organic compounds will not be converted to carbon dioxide inthe U.V. oxidation module. Inorganic carbon species, however, will reactwith the acid from the acidification module to form carbon dioxide. Theeffluent of the U.V. oxidation module 46 is passed into the carbondioxide sensor and the concentration of inorganic carbon species in theaqueous sample is determined using the procedure described above.

After completion of the measurement of the total inorganic carbonconcentration (generally about 5 min.), the electrical power to the U.V.oxidation module 46 is restored by positioning the electrical powerswitch 168 in the ON or CLOSED position. As will be described below,with electrical power, organic compounds present in the aqueous samplewill be converted to carbon dioxide and other products. The effluent ofthe U.V. oxidation reactor 46 will therefore contain carbon dioxide fromboth organic and inorganic compounds and the concentration of totalcarbon species in the aqueous sample is measured by the carbon dioxidesensor 56 as described above. The concentration of total organiccompounds in the aqueous sample is then computed as the differencebetween the total carbon content and the total inorganic carbon content.

As shown in FIGS. 1-4, a major component of the present invention is anU.V. reactor module 46. In this present disclosure, several embodimentsof the U.V. reactor module are described. Each reactor design offerssignificant advantages over the prior art, and each of the embodimentsmay be preferred depending on the particular application of theapparatus. Each design, as will be discussed below, offers advantages interms of simplicity, use of chemical reagent systems, and application tothe wide range of total organic carbon concentrations present in watersamples as diverse as high purity process waters using the electronicsindustry, to municipal and industrial waste waters. Each of the U.V.reactor modules described below can be used in conjunction with theembodiments shown in FIGS. 1-4, depending upon the nature of the aqueoussample stream and the requirements of the analyst.

FIG. 5 is illustrative of a simple U.V. oxidation module for use withthe present invention. The aqueous sample inlet of the U.V. oxidationmodule is in communication with a coiled fused silica tube 172 ofapproximately 120 cm in length and with an internal diameter ofapproximately 1 mm. The radius of the coil is such that a U.V. radiationsource 174 can be positioned in the annular region of the fused silicacoiled tube 172. A suitable power supply and electrical connections (notshown) are used for the operation of the U.V. radiation source 174,which may consist of any known device which emits U.V. radiation, suchas a gas discharge tube or mercury vapor discharge tube. The entireirradiated length of the fused silica coiled tube 172 is packed with ann-type semiconductor coated on a suitable support material 176, held inplace by a retaining system such as quartz wool plugs (not shown) toform the semiconductor-packed photoreactor 177. Any n-type semiconductorwith a band gap greater than about 2 eV may be employed for thisembodiment of the invention, for example TiO₂, SiC, ZnO, CdS. U.V.transparent material, for example silica gel, quartz beads, may be usedas the support. In one embodiment of the U.V. oxidation module, TiO₂particles supported on silica gel were used for the oxidation of a rangeof organic compounds to form carbon dioxide, which was subsequentlymeasured using the carbon dioxide sensor 56. As described in Backgroundof the Invention above, n-type semiconductors are known to serve ascatalysts for the photo-oxidation of organic compounds in aqueoussolution. The design of the U.V. oxidation module shown in FIG. 5 hasbeen demonstrated to provide high efficiency conversion of organiccompounds to form carbon dioxide from aqueous samples at concentrationsup to about 10 mg/L total organic carbon, without the addition of oxygenor other chemical reagents. The simplicity of the design of this U.V.oxidation module is thus a preferred embodiment of the invention for themeasurement of total organic carbon in aqueous sample streams containinglower levels (≦10 mg/L) of organic compounds.

For the determination of total organic carbon in aqueous samples inconcentrations that are above 10 mg/L, the apparatus shown in FIG. 5 maynot be suitable, due to the lack of sufficient concentrations of anoxidizing agent. The measurement range of the present invention can beextended to concentrations greater than 10 mg/L organic carbon if theembodiment of the apparatus shown in FIG. 6 is employed. In thisapparatus, the coiled fused silica tube 172 has been modified to includeshort lengths of palladium tubing 178. The U.V. reactor 179 shown inFIG. 6 is operated under conditions that will result in the formation ofoxygen and hydrogen from the photolysis of water. The in-situ generatedoxygen is then employed, in addition to the semiconductor catalyst 176,in the conversion of organic compounds to form carbon dioxide. Theembodiment of the invention shown in FIG. 6 permits the measurement oftotal organic carbon in aqueous samples at concentrations greater than10 mg/L without the addition of any chemical reagents from externalsources. As noted in the Background of the Invention, hydrogen gasrapidly diffuses through palladium metal and therefore the addition ofshort segments of palladium tubing permits rapid removal of hydrogengenerated from the photolysis of water from the aqueous stream. Ahydrogen exhaust conduit (not shown) is used to remove the gas from theU.V. reactor module.

A third embodiment of the U.V. oxidation module is shown in FIG. 7 andincorporates an electrolysis module 183 for the generation of oxygenprior to the semiconductor catalyzed photoreactor. In FIG. 7, the sampleinlet conduit 184 represents either the aqueous sample stream effluentfrom the inorganic carbon removal module (conduit 42 in FIGS. 1 and 2),the aqueous sample stream effluent from the sample stream splitter(conduit 126 in FIG. 3), or the aqueous sample stream outlet of theacidification module (conduit 166 in FIG. 4). The sample inlet conduit184 is in communication with the aqueous sample inlet of theelectrolysis module 183 which contains a hollow electrolyte permeablemembrane 185. The aqueous sample inlet of the electrolysis module ispositioned such that the sample stream passes through the annular regionof the membrane. A second fluid stream containing ionic compoundssuitable for the conduction of electrical current through solution(electrolyte) is passed into the electrolysis module 183 via conduit 186and this electrolyte solution inlet is positioned such that theelectrolyte solution passes over the exterior of the membrane 185. Inthis embodiment of the U.V. oxidation module, a suitable source of theelectrolyte solution is the water stream(s) from the acidificationmodule 26, the inorganic carbon removal module 48, or an externalaqueous salt solution.

A platinum electrode 187 is positioned in the interior of the hollowmembrane and the outer case of the electrolysis module is constructedfrom a suitable metal 188 and covered with an electrical insulatingmaterial. Electrical connections (not shown) are in contact with theplatinum electrode and the metal case of the module and are connected toa suitable power supply (not shown).

In operation, a electrical potential sufficient to cause theelectrolysis of water is applied between the platinum electrode and themetal case of the electrolysis module. Under these conditions, theplatinum electrode serves as the anode for the generation of molecularoxygen from the aqueous sample in the interior of the hollow membrane.The oxygen-containing sample stream is then passed via conduit 189 tothe inlet of the semiconductor-packed photoreactor 177, described inFIG. 5. The metal case of the electrolysis module serves as the cathodein this electrolysis reaction and hydrogen gas that is generated isremoved from the electrolysis module via the electrolyte outlet 190.

The electrolysis module 183 permits generation of oxygen atconcentrations much greater than the dissolved oxygen levels in aqueoussamples or the concentrations of oxygen that can be generated bysemiconductor-catalyzed photolysis of water. Thus the embodiment of theU.V. reactor module illustrated in FIG. 7 permits the measurement ofhigher concentration levels of total organic carbon in aqueous samplesthan can be achieved using the U.V. reactor modules described in FIGS. 5and 6, but requires the addition of an external power supply and anelectrolyte stream.

The placement of the electrolysis module prior to the U.V. radiationsource offers significant advantages over the disclosure of Winkler. Theuse of a membrane to separate the anode and cathode is incorporated inthe present design and was described by Winkler. However, in Winkler'sdevice, the U.V. radiation source is an integral part of theelectrolysis system. Electrolyte permeable membranes such as Naphion,are known to undergo photo-decomposition upon exposure to U.V.radiation. Thus, in the invention described by Winkler, significantlevels of organic compounds and carbon dioxide are added to the aqueoussample stream from the decomposition of the membrane. In the presentinvention, the membrane is not exposed to U.V. radiation and, therefore,the potential of organic compound contamination of the sample stream iseliminated.

In the embodiments of the U.V. oxidation reactor described, oxidation oforganic compounds to form carbon dioxide is based solely onsemiconductor-catalyzed oxidation combined with photolytic orelectrolytic generation of oxygen. For aqueous samples containing higherlevels of total organic compounds (≧30 mg/L), it is desirable to use anadditional chemical oxidizing agent such as persulfate ion. Aspreviously noted, this reagent is widely used in the prior art, but asnoted above, aqueous solutions of this reagent undergo decompositionand, as a consumable reagent, frequent preparations of aqueous solutionsof persulfate are required for methods and apparatus employing thisreagent for the oxidation of organic compounds to form carbon dioxide.As is also noted above, the reagent can be generated, in-situ, by theelectrolysis of aqueous solutions of sulfate ion. Thus, an improvementover existing devices for the measurement of total organic carboncontent of aqueous solutions can be achieved using a combination ofin-situ generation of the persulfate reagent, coupled with or withoutsemiconductor-catalyzed photo-oxidation.

Accordingly, an additional embodiment of the U.V. oxidation module,shown in FIG. 8, combines in-situ generation of persulfate with thepreviously described semiconductor photoreactor.

In FIG. 8, the sample inlet conduit 191 represents the aqueous samplestream effluent as described for conduit 184 in FIG. 7. A sulfatereservoir 192 is in communication via conduit 194 with one inlet to amixing tee 196, and the aqueous sample stream is in communication with asecond inlet to the mixing tee 196 via conduit 191. The outlet of themixing tee 196 is in communication via conduit 198 with the aqueoussample inlet of the persulfate generation module 200. The outlet of thepersulfate generation module is in communication via conduit 220, withthe inlet of the semiconductor-packed photoreactor 177. The componentsof the persulfate generation module including the hollow membrane,platinum electrode, metal outer casing, electrolyte solution andexternal power supply are the same as described above for FIG. 7. Inoperation, however, the principal electrochemical reactions occurring inthe persulfate generation module are fundamentally different.

As noted in the discussion of prior art, Smit and Hoogland havedemonstrated that application of an electrical potential greater thanabout -2.1 V versus the standard hydrogen electrode to an aqueoussolution of sulfate ions will result in the formation of persulfate,hydrogen peroxide and molecular oxygen at the anode and production ofhydrogen at the cathode. In the embodiment of the invention shown inFIG. 8, a reservoir containing 1 to 3M of a sulfate salt or sulfuricacid is added to the aqueous sample stream prior to entrance into theelectrolysis module. In the electrolysis module, persulfate, hydrogenperoxide and oxygen are produced at the platinum electrode and hydrogengas produced at the outer metal housing (cathode) of the module. Thepersulfate, hydrogen peroxide, and oxygen are transported along with theflowing aqueous sample stream into the semiconductor-packed photoreactor177. This combination of in-situ generated chemical oxidizing agents andsemiconductor-catalyzed photo-oxidation of organic compounds to formcarbon dioxide provides highly efficient conversion of organic compoundsto carbon dioxide, for the determination of total organic carbonconcentrations in aqueous samples as high as 100 mg/L carbon.

The next embodiment of the U.V. reactor module for this invention isshown in FIG. 9. In this design, the effluent of the sulfate ionreservoir 192 is in communication with the electrolysis module 200 viaconduit 221. The operation of the electrolysis module is identical withthat described above for FIG. 3c, however, the concentration of sulfateions entering the electrolysis module is much greater in the designshown in FIG. 9 than shown in FIG. 8, thus resulting in the generationof higher concentrations of persulfate in the electrolysis module. Theoutlet of the electrolysis module is in communication via conduit 222with one inlet of a mixing tee 224. The aqueous sample inlet 184 is incommunication with the second inlet of the mixing tee 224 and the outletof the mixing tee is in communication via conduit 226 with the inlet ofthe semiconductor-packed photoreactor 177.

Using the higher concentration of persulfate generated in the embodimentof the invention shown in FIG. 9, where the electrolysis module isoff-line, determination of the total organic carbon content atconcentrations as great as 1000 mg/L can be achieved.

A final embodiment of the U.V. oxidation module of present invention isshown in FIG. 10. In this embodiment, electrolytic generation of theoxidizing agents is conducted in the U.V. photoreactor chamber. A sampleinlet 228 which represents either the aqueous sample effluent from theinorganic carbon removal module (conduit 42 in FIGS. 1 and 2), theaqueous sample stream effluent from the sample stream splitter (conduit126 in FIG. 3), the aqueous sample stream outlet of the acidificationmodule (conduit 166 in FIG. 4) or the effluent from the sulfate solutionmixing tee (conduit 198 in FIG. 8) is located in the bottom of apressure plate assembly 229. A ceramic spacer plate 230 is in directcommunication with the aqueous sample inlet portion of the pressureplate assembly. A serpentine groove 232, approximately 0.02 inches indepth, is cut into the ceramic spacer to provide a flow path for theaqueous sample through the oxidation module. The outlet of the ceramicspacer is in direct communication with a sample outlet 234 located inthe pressure plate assembly. The sample outlet 234 is in directcommunication with the aqueous sample inlet of the carbon dioxide sensor46 via conduit 52. A matching serpentine gas flow channel 236 ismachined into the pressure plate assembly and is in communication with apurge gas inlet 238 and a purge gas outlet 240. An external purge gassource is in communication with the purge gas inlet 238 and an inert gas(e.g., nitrogen, helium, etc.) or an air source can be employed. Groovesfor the placement of O-rings (not shown) are machined into the pressureplate assembly 242 and the ceramic spacer plate 244 for the purpose ofproviding gas and liquid tight seals in the U.V. reactor module design.A thin sheet of palladium metal 246 is positioned between the pressureplate assembly and the ceramic spacer plate. A fused silica plate 248 ispositioned above the ceramic spacer plate and a platinum metal 250 isdeposited in a matching serpentine pattern on the lower side of thefused silica plate. A U.V. radiation source 252 is positioned directlyabove the fused silica plate, and the entire assembly housed in anelectrically insulated and light-tight container (not shown).

A suitable power supply and electrical connections (not shown) are usedfor the operation of the U.V. radiation source. A second power supplyand electrical connections (not shown), capable of producing voltagesand currents sufficient for the electrolysis of water and for theoxidation of sulfate to persulfate is employed to provide an electricalpotential between the platinum trace on the fused silica plate 250 andthe palladium metal sheet 246.

In operation, the serpentine groove in the ceramic spacer 232 may bepacked with the semiconductor-catalysts described above or may beoperated without the use of a catalyst. A purge gas flow rate of about 1to 10 mL per minute is passed through the gas flow channel 236 and anaqueous sample flow rate of about 10 to 100 microliters per minute ispassed through the sample inlet 228.

As described above, the application of an electrical potential betweenthe platinum anode and the palladium cathode will result in theproduction of oxygen in the presence of water, or hydrogen peroxide andpersulfate in the presence of sulfate ion, at the anode and hydrogen gasat the palladium cathode. The hydrogen gas will rapidly diffuse throughthe thin palladium sheet into the purge gas channel and be rapidlyremoved from the system. The in-situ generated oxidizing agents combinedwith semiconductor-catalyzed photolytic reactions (or uncatalyzed,direct U.V. photolysis) permits rapid oxidation of the organic compoundsto form carbon dioxide.

This embodiment of the present invention offers significant improvementsover the system described by Winkler. In contrast with Winkler's devicewhich requires a solid, polymeric electrolyte, the design of theinvention as described in FIG. 10 contains no organic carbon containingmaterials.

A more detailed description of the carbon dioxide sensor 56 component ofthe present invention is shown in FIG. 11. The carbon dioxide selectivemembrane 59 is positioned between two stainless steel meshes 254. Thesemesh elements support the carbon dioxide selective membrane and alsofacilitate mixing in the two aqueous solutions by producing turbulentflow. As explained in the description of the operation of the carbondioxide sensor in the discussion of FIG. 1, conduit 92 is used totransfer the carbon dioxide containing aqueous stream into themicro-conductivity cell 94 for subsequent measurement. Themicro-conductivity cell includes a conductivity electrode 256 andtemperature sensor 258 used for temperature compensation in theconductivity measurement.

A second embodiment of the carbon dioxide sensor is shown in FIG. 12. Inthis design, conduit 92 has been eliminated and the conductivityelectrode 256 and temperature sensor 258 are an integral part of thecarbon dioxide sensor.

Representative examples of the performance characteristics of the carbondioxide sensor described in FIG. 11 are shown in FIG. 13 and FIG. 14 andin Table 1. As described above, the operation of the sensor 56 is basedon the establishment of an equilibrium across a carbon dioxide selectivegas permeable membrane existing between the aqueous sample stream and adeionized water sample. After this equilibrium has been established(typically 5 minutes), the deionized water sample containing carbondioxide in the form of carbonate and bicarbonate water is swept into themicroconductivity sensor by the introduction of a pulse of deionizedwater from the deionized water module. A plot of the response of themicro-conductivity sensor versus time as the equilibrated water sampleenters the conductivity detector is shown in FIG. 13. As shown theoutput of the conductivity sensor increases rapidly as the sample flowsinto the cell. After a short period of time (approximately 30 seconds),the conductivity reaches its maximum value and remains at about thatvalue for approximately 50 seconds. At this point in the measurementcycle, the conductivity is recorded and used in the calculation of totalorganic carbon concentration or total carbon concentration.

The carbon dioxide sensor has a linear response to the concentration ofcarbon dioxide in the aqueous sample stream as shown in FIG. 14 for theanalysis of aqueous samples containing 0.05 to 125 mg/L of carbon. Thisdata is presented to demonstrate the linearity of the carbon dioxidesensor and does not represent the full range of organic and inorganiccarbon concentrations that can be determined using the presentinvention.

The carbon dioxide membrane 58 employed in the carbon dioxide sensor inone embodiment of the present invention was constructed from aTeflon-like material, perfluoroalkoxy resin ("PFA"). As shown in TableI, the use of this material in the carbon dioxide sensor providessignificantly higher selectivity for the passage of carbon dioxidecompared with other compounds which may be present in aqueous samplesand potentially interfere in the measurement of carbon dioxide using the

                  TABLE 1                                                         ______________________________________                                        Selectivity of permeable membranes:                                           potential interferences in CO.sub.2 measurements                                               DETECTOR RESPONSE                                                             (ppm C)                                                                SPIKE CON-           POROUS*                                        COMPOUND  CENTRATION   PFA     PTFE    Tefzel*                                ______________________________________                                        I.sub.2   13 ppm       ND      --      --                                     HNO.sub.3 1000 ppm     ND      --      --                                     Na.sub.2 SO.sub.4                                                                       1000 ppm     ND      --      --                                     Na.sub.2 SO.sub.3                                                                       21 ppm       ND      1       0.4                                    NaNO.sub.2                                                                              11 ppm       0.2     5       0.6                                    NaCl      1000 ppm     ND      --      --                                     NaOCl     10 ppm       ND      --      --                                     Na.sub.2 S                                                                              15 ppm        0.05   6       0.4                                    Na.sub.2 S                                                                              150 ppm      2.0     30      2.0                                    Formic Acid                                                                             10 ppm       ND      1       0.7                                    Acetic Acid                                                                             10 ppm       ND      1       0.6                                    ______________________________________                                         *From Kobos, et al.                                                           ND: no detectable increase                                                    --: data not reported   conductometric technique described in the             invention. For comparison purposes, data reported by Kobos et al. in 54     Anal. Chem. 1976-1980 (1982) are included in Table I.

A more detailed description of the acidification module 16 is shown inFIG. 15. The filter outlet conduit is used to transport the aqueoussample into the interior of a hollow membrane 260. Aqueous acid fromeither the acid reservoir 18 or the acid/base generation module 110passes through the acid inlet conduit 262, which is the equivalent ofconduit 20 in FIG. 1 or conduit 112 in FIG. 2. The housing for theacidification module is constructed from polyvinylidene difluoride"PVDF" plastic to minimize possible contamination of the aqueous sample.

In summary, the different embodiments of the present invention describedabove represent significant improvements over the methods and apparatusexisting for the measurement of total organic carbon and total carboncontent of aqueous samples. The present invention can be used for thesedeterminations in a wide range of samples, with minimal use of externalchemical reagents. As outlined below, each of the individual componentsof the present invention also offer significant improvements over theprior art.

The use of a carbon dioxide selective membrane and conductometricdetection applied to the measurement of total organic carbon and totalinorganic carbon concentrations in aqueous samples offers these specificadvantages: 1) no purge gas, gas/liquid purge apparatus or drying systemis required, 2) the conductrometric detection system provides excellentlong-term calibration stability and minimal fouling or contaminationsince the sensor is only exposed to carbon dioxide in deionized water,3) the size of the conductivity sensor can be sufficiently small thataccurate measurement in samples as small as 0.1 mL can be achieved, evenat the instrument detection limit, 4) conductrometric detection providesa large linear dynamic range, typically one to three orders of magnitudegreater than other techniques utilized for the measurement of carbondioxide in aqueous samples, 5) the sensitivity of the carbon dioxidesensor and conductivity detector is substantially lower than othertechniques (detection limits approximately 2-5 ug/L of carbon), 6) nosample clean-up or dilution is required, 7) the combination of aninorganic carbon removal module and a carbon dioxide sensor virtuallyeliminates any interference from other volatile gases, and 8) thedetector response is insensitive to changes in sample flow rate.

The U.V. oxidation module of the present invention incorporates severalnew techniques which offer distinct advantages over existing techniquesfor the oxidation of organic compounds to form carbon dioxide. The useof an n-type semiconductor as a catalyst to photo-oxidize organiccompounds for the measurement of organic compounds in water is novel, inthat no external chemical oxidizing reagents are required for samplescontaining up to 10 mg/L of carbon. With the addition of oxygen (eithergenerated photolytically or electrochemically) efficient oxidation canbe achieved for samples containing as high as 30 mg/L of carbon. Thein-situ generation of persulfate ion permits the present invention tomeasure total organic compounds in concentrations as high as 1000 mg/L.

The use of aqueous solution of persulfate is widely used for theoxidation of organic compounds in the determination of total organiccarbon in water. However, this reagent is unstable and fresh solutionmust be prepared, typically once a month. The in-situ generation ofpersulfate using the electrolysis of aqueous sulfate solutions overcomesthis problem. In addition, the electrolysis of aqueous sulfate solutionsalso results in the generation of hydrogen peroxide which is a powerfuloxidizing agent.

Electrolytic generation of oxygen in aqueous solution for thedetermination of total organic carbon is disclosed by Winkler. However,the present invention improves upon the disclosure of Winkler in two keyareas. First we have effected a simplification of the electrolysissystem. Winkler teaches the use of a solid polymer electrolytepositioned between the anode and the cathode. In the present invention,no solid polymer electrolyte is required and the cathode is constructedfrom palladium, which is selectively permeable to molecular hydrogen.This improved design permits the generation of an oxidizing agent at theanode and the instantaneous absorption, diffusion and expulsion ofhydrogen generated at the cathode.

As previously noted, photo-decomposition of the solid polymerelectrolyte will occur in Winkler's device, leading to a backgroundsource of carbon in the system. The simplification of the electrolysismodule by elimination of the solid polymer electrolyte eliminates thisbackground contamination. Another problem inherent in the Winkler designis that the aqueous sample will permeate into the solid polymerelectrolyte. Organic compounds or carbon dioxide dissolved in thisentrained water will eventually diffuse back into the bulk samplecausing at least an increased analysis time, and at worst,cross-contamination between samples resulting in erroneous measurements.

We claim:
 1. A process for the measurement of carbon compounds,comprising: acidifying an aqueous sample in a sample stream containingcarbon dioxide, bicarbonate, carbonate and organic carbon, to convertbicarbonate and carbonate into carbon dioxide; removing the carbondioxide from said sample stream in an inorganic carbon removalmodule;converting organic carbon in said sample stream into carbondioxide in an oxidation reactor; and passing the sample streamcontaining carbon dioxide into carbon dioxide detection means includinga carbon dioxide permeable membrane with two sides, having the samplestream on one side and deionized water on the other side, whereby carbondioxide passes through the membrane from the sample stream to thedeionized water to form water with carbon dioxide in solution; andmeasuring the conductivity and temperature of the dioxide concentrationin the water with carbon dioxide in solution.
 2. The process of claim 1wherein the inorganic carbon removal module includes a gas permeablemembrane through which the carbon dioxide passes.
 3. The process ofclaim 1 wherein the oxidation reactor includes a photo-reactor having:asource of ultra-violet radiation; and an ultra-violet radiationtransparent sample chamber, said sample chamber having a sample streaminlet and outlet.
 4. The process of claim 1 wherein the oxidationreactor includes:an in-line electrolysis cell for the generation ofoxidizing agents from the electrolysis of water; and a photo-reactorhaving: a source of ultraviolet radiation; and an ultra-violet radiationtransparent sample chamber, said sample chamber having a sample streaminlet and outlet.
 5. The process of claim 1 wherein the oxidationreactor includes:means for introducing an aqueous solution containing anoxidizing agent into the sample stream; and a photo-reactor having: asource of ultraviolet radiation; an ultra-violet radiation transparentsample chamber, said sample chamber having a sample stream inlet andoutlet.
 6. The process of claim 1 wherein the oxidation reactor includesa combined electrolysis/photolysis oxidation system for the generationof oxidizing agents and electrochemical and photochemical oxidation oforganic compounds, and further includes:a) a sample chamber constructedfrom an ultra-violet radiation transparent material to permit exposureof the sample stream to ultra-violet radiation; b) an ultra-violetradiation source in radiative communication with said sample stream; c)electrolytic cell means disposed in said sample chamber comprising ananode adjacent to said sample stream, said anode in electricalcommunication with said sample stream, a cathode adjacent to and inelectrical contact with said sample stream, said cathode beingsubstantially impermeable to water and permeable to hydrogen; d) meansfor receiving generated hydrogen adjacent to said cathode; and e) meansfor removing hydrogen from said receiving means.
 7. A process for themeasurement of carbon compounds, comprising:acidifying an aqueous samplein a sample stream containing carbon dioxide, bicarbonate, carbonate andorganic carbon, to convert bicarbonate and carbonate into carbondioxide; converting organic carbon in said sample stream into carbondioxide in an oxidation reactor wherein the oxidation reactor includessections of fused silica tubing connected to lengths of palladium tubingand a source of ultra-violet radiation, wherein said palladium tubingremoves molecular hydrogen; and passing the sample stream containingcarbon dioxide into carbon dioxide detection means including a carbondioxide permeable membrane with two sides, having the sample stream onone side and deionized water on the other side, whereby carbon dioxidepasses through the membrane from the sample stream to the deionizedwater to form water with carbon dioxide in solution; and measuring theconductivity and temperature of the water with carbon dioxide insolution to determine the carbon dioxide concentration in the water withcarbon dioxide in solution.
 8. A process for the measurement of carboncompounds, comprising acidifying an aqueous sample in a sample streamcontaining carbon dioxide, bicarbonate, carbonate and organic carbon, toconvert bicarbonate and carbonate into carbon dioxide;converting organiccarbon in said sample stream into carbon dioxide in an oxidation reactorwherein the oxidation reactor includes: means for adding sulfate ions tothe sample stream; an in-line electrolysis cell for the generation ofperoxydisulfate and other oxidizing agents by electrolysis; and aphoto-reactor having: a source of ultraviolet radiation; and anultra-violet radiation transparent sample chamber, said sample chamberhaving a sample stream inlet and outlet; and passing the sample streamcontaining carbon dioxide into carbon dioxide detection means includinga carbon dioxide permeable membrane with two sides, having the samplestream on one side and deionized water on the other side, whereby carbondioxide passes through the membrane from the sample stream to thedeionized water to form water with carbon dioxide in solution; andmeasuring the conductivity and temperature of the water with carbondioxide in solution to determine the carbon dioxide concentration in thewater with-carbon dioxide in solution.
 9. A process for the measurementof carbon compounds comprising acidifying an aqueous sample in a samplestream containing carbon dioxide, bicarbonate, carbonate and organiccarbon, to convert bicarbonate and carbonate into carbondioxide;converting organic carbon in said sample stream into carbondioxide in an oxidation reactor wherein the oxidation reactor includesmeans for introduction peroxydisulfate to the sample stream; and aphoto-reactor having: a source of ultraviolet radiation; and anultra-violet radiation transparent sample chamber, said sample chamberhaving a sample stream inlet and outlet; and passing the sample streamcontaining carbon dioxide into carbon dioxide detection means includinga carbon dioxide permeable membrane with two sides, having the samplestream on one side and deionized water on the other side, whereby carbondioxide passes through the membrane from the sample stream to thedeionized water to form water with carbon dioxide in solution; andmeasuring the conductivity and temperature of the water with carbondioxide in solution to determine the carbon dioxide concentration in thewater with carbon dioxide in solution.
 10. A process for the measurementof the total organic carbon, total inorganic carbon and total carboncontent of aqueous samples comprising:(a) acidifying a sample streamcontaining an aqueous sample in an acidification module to convertbicarbonate and carbonate in the aqueous sample into carbon dioxide; (b)dividing said stream in a stream splitting device into first and secondstreams; (c) passing said second sample stream containing carbon dioxideformed in the acidification module into a carbon dioxide detector havinga gas permeable membrane selective for the passage of carbon dioxideseparating the sample stream from a deionized water stream; whereincarbon dioxide is diffused across the membrane into the deionized waterto form water with carbon dioxide in solution, and said water withcarbon dioxide in solution is passed into a conductivity and temperaturemeasurement cell for measurement of the total concentration of ionicspecies in, and the temperature of, the water with carbon dioxide insolution, said concentration of ionic species being proportional to theconcentration of inorganic carbon present in the aqueous sample; (d)passing said first sample stream into an oxidation reactor for theconversion of organic compounds to carbon dioxide; said first samplestream from the oxidation reactor passing into a carbon dioxide detectorhaving a gas permeable membrane selective for the passage of carbondioxide separating the sample stream from a deionized water streamwherein carbon dioxide is diffused across the membrane into thedeionized water with carbon dioxide in solution; and said water withcarbon dioxide in solution is passed into a conductivity and temperaturemeasurement cell for measurement of the total concentration of ionicspecies in, and the temperature of, the water with carbon dioxide insolution, said concentration of ionic species being proportional to theconcentration of the total carbon species in the aqueous sample; and (e)calculating the concentration of total organic carbon present in theaqueous sample from the difference in measured total carbon speciesconcentration and total inorganic carbon species concentration.
 11. Aprocess for the measurement of the total organic carbon, total inorganiccarbon and total carbon content comprising:(a) acidifying a samplestream containing an aqueous sample in an acidification module toconvert bicarbonate and carbonate species in the aqueous sample intocarbon dioxide; (b) introducing said sample stream into an oxidationreactor having control means for either permitting complete oxidation oforganic compound in the sample stream to form carbon dioxide orpreventing oxidation of any organic compounds; (c) controlling saidoxidation reactor to prevent oxidation of the organic compounds in thesample stream at a first time and passing the sample stream into anapparatus containing a gas permeable membrane selective for the passageof carbon dioxide separating the sample stream from a deionized waterstream, allowing diffusion of the carbon dioxide from this sample streamacross the membrane into the deionized water to form water with carbondioxide in solution, and passing said water with carbon dioxide insolution into a conductivity and temperature measurement cell formeasurement of the total concentration of ionic species in, and thetemperature of, the water with carbon dioxide in solution, theconcentration of ionic species being proportional to the concentrationof inorganic carbon species present in the aqueous sample; (d)controlling said oxidation reactor such that complete oxidation oforganic compounds occurs in the sample stream at a second time differentthan the first time, and passing the sample stream containing carbondioxide into an apparatus containing a gas permeable membrane selectivefor the passage of carbon dioxide separating the sample stream from adeionized water stream; allowing diffusion of the carbon dioxide fromthe sample stream across the membrane into the deionized water to formwater with carbon dioxide in solution, and passing said water withcarbon dioxide in solution into a conductivity and temperaturemeasurement cell for measurement of the total concentration of ionicspecies in, and temperature of, the water with carbon dioxide insolution, the concentration of ionic species being proportional to theconcentration of the total carbon species in the aqueous sample; and (e)calculating the concentration of total organic carbon species present inthe aqueous sample from the difference in measured total carbon speciesconcentration and total inorganic carbon species concentration.